Method for the noise-free evaluation of radar signals

ABSTRACT

According to the invention a reference measurement is carried out without or with only one measurement object and the frequencies of the noise fraction are determined for the resulting frequency spectrum. Discrete measurement values are determined at equidistant sampling points in the form of complex-value overlays of oscillation functions and the noise and useful frequencies and by means of mathematical calculation methods corrected by the noise fractions. The measurement values corrected in this way are then subjected to a known method of frequency analysis.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on and hereby claims priority to GermanApplication No. 199 25 216.5 filed on 1 Jun. 1999 in Germany, and PCTApplication No. PCT/DE00/01407 filed on 4 May 2000, the contents ofwhich are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The invention relates to the analysis of frequency spectra of afrequency modulated continuous wave (“FMCW”) microwave radar system, inwhich there is at least one virtually constant interference frequency.

In FMCW radar systems which are used, for example, as distance sensors,the parameters of the range and speed of a measurement object areobtained by analysis of the frequencies contained in a radar signal. Allknown frequency analysis methods (for example, that described inWO99/10757) have limited resolution, however, by virtue of the system.This creates a critical difficulty in determining a frequency in theimmediate vicinity of another frequency.

FMCW signals often contain not only the desired useful frequencies f_(N)which are caused by reflections on measurement objects in themeasurement area, but also undesirable systematic interferencecomponents at virtually constant frequencies f_(S), which create acritical difficulty in determining f_(N) in their immediate vicinity. Itis therefore necessary to attempt to suppress these interferencecomponents effectively.

A method for suppressing systematic interference frequencies isdescribed in S. V. Vaseghi: “Advanced Signal Processing and DigitalNoise Suppression”, Wiley Teubner, Chichester 1994. This method assumesthat the frequencies, amplitudes and phases of the interferencecomponents vary from one measurement to the next. DE 43 32 071 A1describes a method in which the interference signal reflected on theantenna is compared with a previously recorded signal profile, in orderto detect offsets occurring at the antenna. This method assumes that thefrequencies, amplitudes and phases of the interference components varyonly due to external influences, such as antenna offsets.

The interference components in FMCW signals may be caused by internalreflections in the electronics and on the antenna, or by externalreflections, for example on the bottoms of containers, and on containerstruts. Irrespective of the particular measurement process, theyactually have virtually constant, a-priori known frequencies f_(s),which can either be measured or can be calculated from the geometry ofthe radar sensor. The associated amplitudes and phases, in contrast,fluctuate severely, for example due to drift in the radar electronics,which changes the radar mid-frequency, and are therefore a-prioriunknown.

SUMMARY OF THE INVENTION

One potential object of the present invention is to specify a method forevaluation of a measurement signal of an FMCW radar system by whichsystematic interference due to fixed-position reflectors, which causeinternal or external reflections, can be effectively eliminated.

The method according is based on evaluation of the radar signal byfrequency spectrum analysis. The interference frequencies which arepresent in the spectrum and result from interference reflections areeliminated by specific mathematical methods. For this purpose, areference measurement is first of all carried out which, depending onthe nature of the interference frequencies to be eliminated, is carriedout without or with only one measurement object as the reflectiontarget. The frequency spectrum resulting from the reference measurementis preferably analyzed with a greater frequency resolution than is usedin normal measurement operation of the apparatus. The frequencies of theinterference components in the radar signal are determined in this way.The method may be based on an adaptive least-squares-fit determination,matched to the individual measurement signal, of the interferenceamplitudes and phases for the known interference frequencies. Theinterference signal, adapted from one measurement to the next, is thuscalculated and is subtracted from the FMCW measurement signal. The FMCWmeasurement signal that has been cleaned up in this way is thensubjected to one of the known frequency analysis methods. Anotherpossibility for determining the interference components is toreconstruct them mathematically, on the basis of the system design.

Discrete values relating to the measurement signals are determined at apredetermined number of sampling points, which are offset by the sametime interval with respect to one another. The values detected in thisway are recorded as complex-value superimpositions of oscillationfunctions at the interference and useful frequencies, and have theinterference components removed from them by mathematical computationmethods. The measured values that have been cleaned up in this way arethen subjected to one of the known frequency analysis methods.

BRIEF DESCRIPTION OF THE DRAWING

The following text contains a more detailed description of examples ofthe method according to the invention with reference to the attachedFIGURE, which shows one layout of a microwave radar system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to like elementsthroughout.

The FIGURE shows a monostatic FMCW radar system, in which the signalsource is a tunable oscillator 10 (VCO). The signal frequency ispreferably swept linearly from a lowest value to a highest value, orvice versa. The signal passes through a transmitting/receiving duplexer11, which, for example, can be formed by a circulator or directionalcoupler, and is passed to the antenna 12. The transmitting/receivingduplexer 11 is provided to separate the transmitted signal from thereceived signal. The received signal is supplied to a mixer 13, in whichit is mixed with the transmitted signal for demodulation purposes. Thedown-mixed signal is then supplied to the evaluation unit 16, preferablyafter filtering, which is not shown, to eliminate high-frequencyinterference components. The calculation steps which characterize themethod are described in detail further below and are also carried out inthe evaluation unit. Two signals are subtracted in one of these methods.The subtraction process can then either be carried out directly,digitally in the evaluation unit, or in analog form by the adder 14. Inthe case of analog subtraction, the modulated received signal first ofall passes through this adder and is preferably digitized by an AIDconverter 15, so that it can be evaluated in an evaluation unit 16formed by a microprocessor (μP). The values which represent theinterference signals and are intended to be subtracted from thedemodulated received signal are converted, for example by a D/Aconverter 17, to an analog signal which is supplied, inverted (in itsnegative form), to the adder 14.

In the method according to the invention, a reference measurement iscarried out first of all. This reference measurement is used todetermine the frequencies, which are assumed to be virtually constant,of the interference reflections. When recording the referencemeasurement, the occurrence of useful frequencies in the directimmediate vicinity of the interference frequencies is avoided. If theradar system is used, for example, as a level measurement device,interference frequencies which occur can also be caused by reflections,which do not vary with time, on the tank fittings in the container. Inthis case, measurements are carried out in the empty container, withoutany filling level in it. The accuracy of the reference measurements canbe increased by averaging over a number of measurements, by usingfilters or else by measurement using an FMCW measurement bandwidth whichis wider than that for normal operation. During subsequent, normalmeasurement operation, the associated complex amplitudes of theinterference signal terms in the measurement signal are then determinedon the basis of the knowledge of these interference frequencies, theassociated complex amplitudes of the interference signal terms in themeasurement signal. This is preferably done by using linear algebramethods to apply a method for minimizing the square of the error of thesolution of an overdefined equation system (least-squares-fit) to alinear transformation of a measurement signal sampled at equalintervals.

The analog radar signal is sampled at N sampling points, which areoffset by the same time interval with respect to one another, so thatthis results in N discrete values, which form the components of avector, in chronological sequence. The N components x(n), n=1, 2, 3, . .. , N−1, N, are written, in complex notation, as a superimposition of pexponential terms with white noise:${x(n)} = {{\sum\limits_{k = 1}^{p}{C_{k}^{\quad \omega_{k}n}}} = {{\omega (n)}.}}$

In this case, k is a numbering for the various frequencies ω_(k) thatoccur, with the complex amplitudes c_(k). The summation extends over allp frequencies contained in the signal. Since the interferencefrequencies are known from the reference measurement, the complexamplitudes of the interference components can be determined usingapproximate linear algebra methods in the spectrum of the measurementsignal, such that the interference components can very largely beeliminated. This determination of the complex amplitudes using theleast-squares principle (least error-squares method, that is to say theminimum sum of the squares of the errors) has considerably higherresolution than the frequency estimation methods, that is to sayinterference components are still determined correctly even if there arevery closely adjacent useful frequencies.

The matrix F is preferably formed in order to determine the complexamplitudes contained in the normal measurement signal, in which matrix Fthe number p of rows corresponds to the number of interferencefrequencies present, and which matrix has the same number of columns asthe number of sampled measured values. Each point in the matrix containsthe exponential function of the product of i=−1, the column number andthe respective interference frequency associated with a row. Thiselement is thus in the form e^(jω) _(k) ^(n) where k is the number ofthe interference frequency and the number of the row in the matrix, andn is the number of the column with a value from 1 to N:$F = \begin{bmatrix}^{j\quad \omega_{1}1} & ^{j\quad \omega_{1}2} & \ldots & ^{j\quad \omega_{1}N} \\^{j\quad \omega_{2}1} & \ldots & \quad & \vdots \\\vdots & \quad & \ldots & \vdots \\^{j\quad \omega_{p}1} & \ldots & \ldots & ^{j\quad \omega_{p}N}\end{bmatrix}$

The vector of the measured signal is multiplied by the pseudo-inversiveof the above matrix, which can be determined by applying a least-squaresmethod for determining the solution of an overdefined linear equationsystem to the previously specified matrix. This results in the complexamplitudes Ck associated with the exponential functions of theinterference frequencies, where k now represents the sequentialnumbering of the interference frequencies. The interference componentwhich has been approximated by calculating the associated complexamplitudes can then be subtracted from the components of the vector ofthe measurement signal. This thus results in the vector, from which thesystematic interference component has been removed, of the values of themeasurement signal at equal time intervals. Conventional frequencyanalysis methods can then be applied to this. In particular in the caseof measurements with FMCW radar sensors in the near area, this methodleads to an improvement in the measurement accuracy. Furthermore, thismethod is not sensitive to drift in the electronics or to interferencefrom closely adjacent frequencies.

The invention has been described in detail with particular reference topreferred embodiments thereof and examples, but it will be understoodthat variations and modifications can be effected within the spirit andscope of the invention.

What is claimed is:
 1. A method for elimination of interferencecomponents at constant frequencies in a frequency spectrum of afrequency modulated continuous wave radar system, comprising: performinga reference measurement of interference signals, in which the occurrenceof useful signals is avoided, determining a spectrum of interferencefrequencies from this reference measurement, measuring a useful signalto produce a measurement signal, sampling the measurement signal at eachof a predetermined number of sampling points which are offset by thesame time interval with respect to one another, using the interferencefrequencies of the frequency spectrum to determine, from the sampledmeasurement signal, complex amplitudes which approximate the proportionof the frequency spectrum of the sampled measurement signals caused bythe interference frequencies, and subtracting the frequency spectrumcaused by interference frequencies from the sampled measurement signalto form a difference signal, wherein in determining the complexamplitudes, a vector which contains samples of the measurement signal ina chronological sequence as components is transformed by a matrix, whichrepresents a pseudo-inverse, which is formed by a least-squares methodfor minimizing the square of the error of the solution of an overdefinedlinear equation system, the matrix has a number of rows corresponding tothe number of interference frequencies, the matrix has a number ofcolumns corresponding to the number of sampling points, and elements ineach row of the matrix are exponential functions of a respectiveinterference frequency multiplied by the square root of −1 and thecolumn number.
 2. A method as claimed in claim 1, wherein the referencemeasurement is performed by passing a reference signal through a medianso that it does not reflect off an object whose properties are to bemeasured.
 3. A method as claimed in claim 2, wherein the useful signalis measured by obtaining returned radar signals with an intended targetpresent.
 4. A method as claimed in claim 1, wherein the useful signal ismeasured by obtaining returned radar signals with an intended targetpresent.
 5. A device to eliminate interference components at constantfrequencies in a frequency spectrum of a frequency modulated continuouswave radar system, comprising: a first measurement unit to perform areference measurement of interference signals, in which the occurrenceof useful signals is avoided; a spectrum unit to determine a spectrum ofinterference frequencies from this reference measurement; a secondmeasurement unit to measure a useful signal to produce a measurementsignal; a sampling unit to sample the measurement signal at each of apredetermined number of sampling points which are offset by the sametime interval with respect to one another; an amplitude determinationunit to use the interference frequencies of the frequency spectrum todetermine, from the sampled measurement signal, complex amplitudes whichapproximate the proportion of the frequency spectrum of the sampledmeasurement signals caused by the interference frequencies; and asubtraction unit to subtract the frequency spectrum caused byinterference frequencies from the sampled measurement signal to form adifference signal, wherein in determining the complex amplitudes, avector which contains samples of the measurement signal in achronological sequence as components is transformed by a matrix, whichrepresents a pseudo-inverse, which is formed by a least-squares methodfor minimizing the square of the error of the solution of an overdefinedlinear equation system, the matrix has a number of rows corresponding tothe number of interference frequencies, the matrix has a number ofcolumns corresponding to the number of sampling points, and elements ineach row of the matrix are exponential functions of a respectiveinterference frequency multiplied by the square root of −1 and thecolumn number.
 6. A method for elimination of interference components atconstant frequencies in a frequency spectrum of a frequency modulatedcontinuous wave radar system, comprising: performing a referencemeasurement of interference signals, in which the occurrence of usefulsignals is avoided, determining a spectrum of interference frequenciesfrom this reference measurement, measuring a useful signal to produce ameasurement signal, sampling the measurement signal at each of apredetermined number of sampling points which are offset by the sametime interval with respect to one another, using the interferencefrequencies of the frequency spectrum to determine, from the sampledmeasurement signal, complex amplitudes which approximate the proportionof the frequency spectrum of the sampled measurement signals caused bythe interference frequencies, and subtracting the frequency spectrumcaused by interference frequencies from the sampled measurement signalto form a difference signal, wherein the reference measurement isperformed by passing a reference signal through a median so that it doesnot reflect off an object whose properties are to be measured.
 7. Themethod as claimed in claim 6, wherein in determining the complexamplitudes, a vector which contains samples of the measurement signal ina chronological sequence as components is transformed by a matrix, whichrepresents a pseudo-inverse, which is formed by a least-squares methodfor minimizing the square of the error of the solution of an overdefinedlinear equation system, the matrix has a number of rows corresponding tothe number of interference frequencies, the matrix has a number ofcolumns corresponding to the number of sampling points, and elements ineach row of the matrix are exponential functions of a respectiveinterference frequency multiplied by the square root of −1 and thecolumn number.
 8. A method as claimed in claim 6, wherein the usefulsignal is measured by obtaining returned radar signals with an intendedtarget present.
 9. A method as claimed in claim 6, wherein the usefulsignal is measured by obtaining returned radar signals with an intendedtarget present.
 10. A device to eliminate interference components atconstant frequencies in a frequency spectrum of a frequency modulatedcontinuous wave radar system, comprising: a first measurement unit toperform a reference measurement of interference signals, in which theoccurrence of useful signals is avoided; a spectrum unit to determine aspectrum of interference frequencies from this reference measurement; asecond measurement unit to measure a useful signal to produce ameasurement signal; a sampling unit to sample the measurement signal ateach of a predetermined number of sampling points which are offset bythe same time interval with respect to one another; an amplitudedetermination unit to use the interference frequencies of the frequencyspectrum to determine, from the sampled measurement signal, complexamplitudes which approximate the proportion of the frequency spectrum ofthe sampled measurement signals caused by the interference frequencies;and a subtraction unit to subtract the frequency spectrum caused byinterference frequencies from the sampled measurement signal to form adifference signal, wherein the reference measurement is performed bypassing a reference signal through a median so that it does not reflectoff an object whose properties are to be measured.